(a) Field of the invention
The present invention relates to a process for continuously manufacturing alcohol from a sugar-containing liquid by using an immobilized microorganism which is obtained by immobilizing an alcohol producing microorganism on a carrier.
(b) Description of the prior art
Alcohol (ethanol) has hitherto been manufactured by the fermentation method and has been used for such wide purposes as drinks, medicines, toiletries and the like. Additionally, people have begun to entertain some apprehension about the exhaustion of fossil fuels, in particular, petroleum. Various studies have been carried out for the purpose of utilizing alcohol as an alternate energy source for petroleum. Therefore, the demand for establishment of an alcohol fermentation method which can meet the requirements for high alcohol productivity and energy conservation has increased.
The fermentation method which can satisfy these requirements should enhance the volumetric efficiency of the fermenter (alcohol productivity per unit volume of the fermenter and unit time) and simultaneously reduce energy consumption taking into consideration not only the fermentation step but also the alcohol-separating step and the waste liquid-treating step. However, such an efficient and economical alcohol fermentation method has not been proposed yet.
In alcohol fermentation, although there are some differences between the microorganisms to be employed, it may be generally stated that as the alcohol concentration in the fermenter increases fermentation speed is largely decreased by the presence of alcohol and other metabolic products of the fermentation process. For instance, when using an alcohol producing yeast, a serious hindrance to the fermentation speed is detectable when the alcohol concentration reaches approximately 7 to 8 wt.% or more, thereby decreasing alcohol formation. In view of this, it was inevitable that conventional methods carry out fermentation in a low alcohol concentration solution. Such a method brings about the problem of treating a large amount of fermentation waste liquid and the necessity of recovering the alcohol from the low alcohol concentration solution. Accordingly, this method is defective because considerably large amounts of energy must be consumed in solving those problems.
As the means for solving those problems of the above-described process, there has been proposed a method which comprises treating the fermentation liquid taken out of the fermenter under reduced pressure to thereby separate the alcohol from the liquid. For instance, Japanese Laid Open Patent Application No. 21592/1981 and Japanese Laid Open Patent Application No. 2685/1982 disclose methods wherein the fermentation liquid taken out of the fermenter is continuously divided into alcohol-containing steam and yeast liquid at a temperature which exerts no influence on the yeast allowing under reduced pressure, and the yeast liquid to be cycled back to the fermenter. These methods are intended to reduce the treatment of the residual fermentation liquid discharged out of the system and to utilize the yeast liquid again in the fermenter.
In removing the alcohol from the fermentation liquid and returning the yeast liquid to the fermenter for re-use, it is necessary that the temperature of the fermentation liquid introduced into the reduced pressure system be lower than the temperature which exerts an influence upon the yeast. In order that the alcohol fermentation may be effected according to this method it is necessary, because the alcohol concentration in the fermenter is low, to further reduce the pressure in the separation step or increase the amount of the yeast liquid which is cycled. Therefore, this method is unavoidably followed by an increase in energy consumed in maintaining the reduced pressure state or in cycling the liquid, and should not be said to be economical.
Japanese Laid Open Patent Application No. 64790/1981 has proposed a method which comprises taking the yeast-containing fermentation liquid out of the fermenter, dividing this liquid into an enriched yeast flow and a yeast-free flow by means of a centrifugal separator or the like, cycling the enriched yeast flow to the fermenter, heating the yeast-free flow to a desired temperature and then introducing it to the evaporation unit device under reduced pressure. However, this method is not always preferable is that not only is the construction of such a unit complicated, but also the cost of equipment, is high particularly for a centrifugal separator capable of treating a large amount of the fermentation liquid, and further, the operating costs are very expensive.